1. Field of the Invention
The present invention relates to an optical shifter that can be used to physically shift, displace, or change the location at which an optical output signal is presented to another location in a head mounted display (HMD) or a projection type display system (i.e., projector) and also relates to an optical display system including such an optical shifter.
2. Description of the Related Art
A liquid crystal display (LCD) includes a pair of substrates and a liquid crystal layer that is sandwiched between the substrates. Multiple pixel electrodes are regularly arranged in columns and rows (i.e., in matrix) on one of the two substrates. A drive voltage, representing an image signal, is applied to each of the pixel electrodes. The optical properties of the liquid crystal layer change on a pixel-by-pixel basis upon the application of this voltage. Thus, an image, character and so on can be displayed on the LCD.
The methods of applying different drive voltages to the respective pixel electrodes on the substrate independently include a “simple-matrix addressing” method and an “active-matrix addressing” method.
In the active-matrix addressing method, multiple switching elements are provided for the respective pixel electrodes on the substrate. A substrate including those switching elements thereon is normally called an “active-matrix substrate”. On the active-matrix substrate, each of those switching elements selectively turns ON or OFF to electrically connect or disconnect its associated pixel electrode to/from its associated signal line. A metal-insulator-metal (MIM) element or thin-film transistor (TFT) may be used effectively as such a switching element.
In its OFF state, the switching element needs to have the highest possible electrical resistance. However, if the switching element in OFF state is exposed to intense radiation, then the electrical resistance of the switching element decreases to generate leakage current. As a result, the electrical charge that has been stored in its associated pixel electrode is lost partially. Also, in that case, a drive voltage at an appropriate level cannot be applied to the pixel electrode. Then, the LCD cannot conduct the display operation as intended. For example, even in its black display mode, the LCD leaks some light unintentionally to decrease the resultant contrast ratio thereof.
In an LCD of a transmission type, an opaque layer, which is often called a “black matrix”, is provided either over the active-matrix substrate or over a counter substrate, which faces the active-matrix substrate by way of the liquid crystal layer, to overcome these problems. However, when the black matrix is provided, the aperture ratio (i.e., the ratio of the total transmitting area to the overall display area) of pixels decreases adversely. To increase the definition sufficiently by reducing the total area of the black matrix, the switching elements or interconnection lines may be downsized. In that case, however, the driving force may decrease or the wiring resistance may increase. Furthermore, it is currently difficult to further reduce the sizes of the switching elements or interconnection lines considering various constraints on the actual manufacturing process of LCDs.
A technique of optically shifting or displacing the image by a distance that is approximately equal to a pixel pitch for the purpose of increasing the screen resolution by utilizing the non-display areas on the black matrix is disclosed in U.S. Pat. No. 4,984,091. According to this technique, as the pixels are shifted, an image displayed is also shifted to a location corresponding to the pixels shifted. Thus, the apparent number of pixels increases, and therefore, even a low-resolution display panel can also display an image having a definition comparable to that of a high-resolution display panel.
U.S. Pat. No. 6,061,103 discloses a method of getting each set of three pixels, representing the three primary colors of red (R), green (G) and blue (B) (which will be herein referred to as “R, G and B pixels”, respectively), displaced optically by an optical shifter one after another and then displaying a superimposed, composite image consisting of three image components represented by the three types of pixels shifted. In this method, the R, G and B pixels are displayed in an area corresponding to one pixel by a time-sequential technique. Accordingly, the apparent resolution can be tripled without reducing the pixel pitch on the display panel.
U.S. Pat. No. 6,061,103 also discloses an optical shifter, including a liquid crystal cell and a birefringent element in combination, as a means for displacing the image optically. The birefringent element is made of a material that refracts incoming light in a different direction depending on the polarization direction of the incoming light. Thus, if the polarization direction of the light that is going to enter the birefringent element has been changed by the liquid crystal cell, then the optical axis of the light (i.e., propagation direction of the light) that is leaving the birefringent element can be shifted.
FIG. 1 illustrates a known optical shifter. As shown in FIG. 1, this optical shifter includes a liquid crystal cell 7 and a birefringent element 11, which are arranged in series in the direction in which incoming light is propagated. The liquid crystal cell 7 may switch from the state of rotating the plane of the electric vector of incoming linearly polarized light (which will be herein referred to as the “plane of polarization”) by 90 degrees to the state of transmitting the incoming linearly polarized light as it is without rotating the plane of polarization thereof at all, or vice versa. The birefringent element 11 can shift the incoming light in accordance with the direction of the plane of polarization of the incoming linearly polarized light.
In the example illustrated in FIG. 1, the direction of the electric vector (i.e., the polarization direction) of the light that is going to enter the liquid crystal cell 7 is the direction coming out of the paper. The liquid crystal cell 7 uses a twisted nematic mode liquid crystal material (which will be herein referred to as a “TN mode liquid crystal material”) with positive refractive index anisotropy Δε. Accordingly, while no voltage is being applied to the liquid crystal layer of the liquid crystal cell 7 (which state will be herein referred to as a “voltage-OFF state”), the liquid crystal molecules thereof are twisted by 90 degrees. Due to the optical rotatory property of the liquid crystal molecules, the plane of polarization of the incoming light is rotated by 90 degrees. On the other hand, while a voltage that is equal to or higher than a predetermined level is being applied to the liquid crystal layer of the liquid crystal cell 7 (which state will be herein referred to as a “voltage-ON state”), the orientation directions of the liquid crystal molecules are aligned with the direction of the electric field generated. Accordingly, the incoming light goes out of the liquid crystal cell 7 without getting its plane of polarization rotated by the liquid crystal molecules. That is to say, the plane of polarization of the outgoing light still crosses the paper at right angles. Then, the birefringent element 11 directly transmits the light with the plane of polarization crossing the paper at right angles but refracts, or shifts, the light with the plane of polarization parallel to the paper.
In the optical shifter shown in FIG. 1, the liquid crystal cell 7 thereof needs to be switched appropriately and quickly from the state of passing, or transmitting, the first linearly polarized light into the state of letting go the second linearly polarized light, having a plane of polarization that crosses that of the first linearly polarized light at right angles, or vice versa, depending on the magnitude of the voltage applied thereto.
As described above, in a liquid crystal cell made of a TN mode liquid crystal material, while no voltage is being applied to the TN mode liquid crystal material, incoming linearly polarized light goes out of the liquid crystal cell as linearly polarized light that has had its plane of polarization rotated by 90 degrees. However, when a voltage is applied to the TN mode liquid crystal material, the orientation directions of the liquid crystal molecules change quickly responsive to the electric field generated. As a result, the liquid crystal layer soon enters the state of not changing the polarization direction of the incoming light. If the voltage that has been applied to the TN mode liquid crystal material is stopped, the liquid crystal molecules recover their original state (i.e., relaxed) but the response speed is not so fast.
Thus, depending on whether the voltage applied to the liquid crystal layer is increased from Low level (typically 0 volts) to High level (typically 10 volts) or decreased from High to Low, the orientation directions of the liquid crystal molecules change at different speeds. That is to say, the liquid crystal molecules have different response speeds in these two situations. To estimate this response speed, a pair of polarizers may be disposed in front of and behind the liquid crystal layer so that their axes cross each other at right angles, and a variation in the transmittance of the liquid crystal layer with time may be measured. FIG. 2 is graph showing how the transmittance of the liquid crystal layer changes if the voltage applied thereto is decreased from High to Low level a predetermined time after the voltage was increased from Low to High level. A time it takes for the transmittance of the liquid crystal layer to decrease from its maximum value to zero will be herein referred to as a “liquid crystal fall response time τ d” while a time it takes for the transmittance to increase from zero to the maximum value will be herein referred to as a “liquid crystal rise response time τ r”. As shown in FIG. 2, the liquid crystal rise response time τ r is relatively short but the liquid crystal fall response time τ d is relatively long. If the liquid crystal fall response time τ d is long, then the image cannot be shifted synchronously with the switching of image components to be displayed on a display panel. Before this problem is described fully, an image switching rate of a display panel will be described.
Normally, a display panel is driven either by an interlacing scanning technique or a noninterlacing scanning technique. In the interlacing scanning, odd-numbered and even-numbered lines are alternately selected on a field-by-field basis. That is to say, if odd-numbered lines are selected for one field (or subframe) of an image, then even-numbered lines are selected for the next field of the image. In this manner, one complete image (or picture), obtained by combining the odd-numbered field and the even-numbered field with each other, is presented on the display. According to this method, each field is normally selected for about 16.6 ms (i.e., at a refresh rate of about 60 Hz). In the noninterlacing scanning on the other hand, multiple lines of the image are sequentially selected one after another, no matter whether the line selected is odd-numbered or even-numbered. As in the interlacing scanning method, each field is also normally selected for about 16.6 ms (i.e., at a refresh rate of about 60 Hz). As used herein, “one field period” refers to one vertical sync period of an image, no matter whether the scanning method adopted is interlacing or noninterlacing. In an LCD, one field period corresponds to a scan period including the blanking interval.
According to the method disclosed in U.S. Pat. No. 6,061,103 identified above, one field period is divided into three subfield periods for the three locations to which the R, G and B pixels are shifted, and three different components of one image (which will be herein referred to as “image subfields” or “subfields” simply) are sequentially presented on the display panel in those three subfield periods. In that case, one subfield period is about 5 ms. Thus, the optical shifter needs to optically displace those image components (or subfields) at as short time intervals as about 5 ms. Furthermore, the shifting of the subfields by the optical shifter should be timed (or synchronized) with the switching of the same subfields on the display panel. Accordingly, no sooner have the subfields been switched on the display panel than the optical shifter must change its states responsive to the voltage applied to the liquid crystal cell.
Actually, though, it is difficult for any liquid crystal cell currently available to change its states quickly enough responsive to the voltage applied thereto. In a liquid crystal cell made of the TN mode liquid crystal material, for example, the liquid crystal rise response time τ r thereof is relatively short but the liquid crystal fall response time τ d thereof is normally about 10+ ms, which is much longer than one subfield period of about 5 ms, as shown in FIG. 2.
Such a difference in response time is created for the following reasons. Specifically, as shown in FIG. 2, the transmittance is increased by applying a voltage to the liquid crystal layer such that the orientation directions of the liquid crystal molecules are aligned with one direction upon the application of the external energy (i.e., the voltage). On the other hand, the transmittance is decreased by stopping the application of the voltage to the liquid crystal layer so that the liquid crystal molecules recover their original orientation states by themselves.
If the liquid crystal material adopted has such a long fall response time τ d, then the polarization directions cannot be switched appropriately, either. This problem will be further described with reference to FIG. 1. As shown in FIG. 1, if the voltage applied to the liquid crystal layer of the liquid crystal cell 7 is changed from ON state into OFF state, then the plane of polarization of the light going out of the liquid crystal cell 7 rotates by 90 degrees. As a result, the optical axis of the light that is leaving the birefringent element 11 shifts from the location B to the location A. In this case, if the fall response time τ d of the liquid crystal material is too long, then the linearly polarized light changes into elliptically polarized light while the liquid crystal molecules are falling. Thus, the same image component will be displayed at both of the two locations A and B and a ghost image will be presented during the interval. Consequently, the resolution of the image decreases.
Also, if there is a big difference between the rise and fall response times τ r and τ d of the liquid crystal material, then the intensity of the ghost image that is produced during the optical shifting of the image from the location A to the location B becomes different from that of the ghost image produced during the optical shifting from the location B to the location A. As a result, a quite perceivable flicker is produced.
Japanese Laid-Open Publication No. 1-191123 discloses an optical shutter that includes dextrorotatory and levorotatory TN mode liquid crystal materials in combination. This optical shutter can be activated at a relatively high rate but cannot be used as an optical shifter. The reason is as follows. The optical shifter needs to shift one of multiple image subfields after another synchronously with switching of those subfields on the display panel, whereas the optical shutter cannot display the subfields continuously because the optical shutter temporarily blocks the optical path mechanically during the operation thereof. That is to say, the optical shutter temporarily suspends the display of one of those subfields during the subfield period thereof.